perf: Broadcast without materialization in concat_arr

[nameexhaustion]

We were accidentally materializing due to using as_materialized_series() / c.array() (which underneath does .as_materialized_series().

Also adjusts some slicing operations for the Scalar column variant to avoid taking refcounts to materialized Series when slicing to 0 or unit-length.

[codspeed-hq]

CodSpeed Performance Report

Merging #20681 will improve performances by 34.72%

_{Comparing nameexhaustion:concat-arr-scalar-opt (9d7f1c3) with main (dd9a180)}

Summary

⚡ 2 improvements
✅ 39 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	nameexhaustion:concat-arr-scalar-opt	Change
⚡	test_groupby_h2oai_q6	12 ms	9.4 ms	+27.29%
⚡	test_pdsh_q21	190.6 ms	141.5 ms	+34.72%

[codecov]

Codecov Report

Attention: Patch coverage is 92.06349% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.03%. Comparing base (0c3c89a) to head (9d7f1c3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-ops/src/series/ops/concat_arr.rs	91.52%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20681      +/-   ##
==========================================
+ Coverage   79.00%   79.03%   +0.03%     
==========================================
  Files        1559     1559              
  Lines      221216   221237      +21     
  Branches     2529     2529              
==========================================
+ Hits       174765   174862      +97     
+ Misses      45872    45794      -78     
- Partials      579      581       +2     

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